A gas turbine is an example of apparatuses that uses a large number of components, such as heat-resistant steel components, superalloy components, and the like. In the gas turbine, wing-shaped blades are arranged on outer peripheries of a plurality of ring-shaped turbine disks, and an axial flow of fluid (i.e., a flow in an axial direction of a rotational shaft) is converted into rotary motion to generate power. Air is taken in from a front of the gas turbine and is then compressed by multistage axial flow compressing units which are downstream thereto. Furthermore, gas including a mixture of compressed air and fuel is burned in a combustion chamber which is further downstream thereto, to generate high-temperature and high-pressure combustion gas. This combustion gas collides with the blades which are mounted on each turbine disk, while axially flowing in a flow path on the outer periphery of each turbine disk, and this axial motion is converted into rotary motion so that each turbine disk is turned at high speed. Driving force generated by this rotation turns the turbine disk which is before the others thereof, via the rotation axis, and further compresses air so as to provide a continuous turn of the turbine disk.
In recent years, in view of saving energy, it is an important technical demand to improve efficiency of the gas turbine. However, the maximum temperature of the combustion gas to be processed is increased by improvement in the efficiency, and therefore, it is necessary for the gas turbine to be capable of operating at higher temperatures. On the other hand, the turbine disks and the blades in the gas turbine are used while being rotated at high speed, and therefore, there is a problem in that they are subjected to high loads which are applied due to centrifugal force during operation of the gas turbine. In addition, the turbine disks and the blades are exposed to high temperature gas of 600 degrees C. or more, and they are used at locations close to the flow path for the high temperature gas. Therefore, it is absolutely necessary for the turbine disks and the blades to have high strength in a high temperature environment. Furthermore, if they are used in an operation pattern in which start and stop of the operation of the gas turbine occurs intermittently, components of the turbine disks and the blades are subjected to repeatedly applied loads, and as a result, thermal stress which occurs at stages of increasing the temperature and of cooling the components, also repeatedly act on them. Accordingly, it is important for the gas turbine to be configured by components having strength sufficiently high against loads and thermal stress that are repeatedly applied as described above.
In addition, there is a tendency that in order to increase efficiency of the gas turbine, dimensions of rotational bodies, such as the turbine disks and the blades are increased, and therefore, a ring formed of a high-quality material, such as heat resistant steel, superalloy, and the like which can resist higher centrifugal force, is needed. In order to accommodate the above demands, in the inside of the gas turbine, a Ni-based superalloy of which typical examples include austenitic heat resistant steel, ferritic heat resistant steel, and a 718 alloy, are primarily used as the heat resistant steel having high strength in high temperature environments.
It has been known that in the Ni-based superalloy having particularly excellent strength in high temperatures (for example, the 718 alloy), among such abovedescribed alloys, fatigue strength thereof can be improved by refining a metal crystal structure of the material thereof. In addition, various techniques have been suggested for a method of reducing size of grains in the inside of the material. For example, as shown in Patent Literature 1, it has been known that in order to refine the crystal structure, a method of precipitating grains which inhibit coarsening of crystal grains, and the like, are advantageous. In addition, as shown in Patent Literature 2, a method is suggested, in which the material is distorted during hot working so as to promote the phenomenon of refining the grains so that fine grains can be obtained. Regarding a method of producing a ring to be used in high temperature environments, a Ni-based superalloy is more expensive than a normal steel material because the Ni-based superalloy mainly includes rare metals. Therefore, near-net-shape forging is often used, in which a near-net-shape material having a shape close to the finished shape, is used as the material to be cut, and it is further cut so that the amount of chips generated during cutting, is reduced, and as a result, production costs can be reduced.
For the near-net-shape forging, hot forging is generally used. As an example of the production process described above, the hot forging process is used, in which upset forging is applied to a columnar billet so as to shape it in a disk shape, a center portion of the disk-shaped columnar billet is punched, ring rolling is applied to the punched columnar billet so as to shape it as a ring having predetermined diameters, and finally, the ring is shaped so as to have a desired shape of a section thereof, by using a die.